The effects of alloying on stress induced void formation in aluminum-based metallizations

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Abstract
Evaporated metallizations composed of aluminum alloys and titanium underlayers were patterned, passivated with plasma enhanced chemical vapor deposited SiNx and aged for 1000 h at 150 °C in order to observe stress-induced void formation. Metal films were analyzed using scanning electron microscopy, transmission electron microscopy, and secondary ion mass spectrometry. The addition of copper to aluminum results in fewer voids than in comparable noncopper metallizations. For Al–Cu films, fewer voids were observed in 1.5 μm lines compared to 5 μm lines, apparently due to the presence of greater stress gradients in the wide line case. Silicon appears to promote void formation by rapid grain boundary diffusion to precipitates. Oxygen incorporation in aluminum produces small grained films, thereby generating many void nucleation sites. High oxygen concentrations produce films with a few long, channel-like voids and many small voids. Titanium underlayers decrease the percent of metal volume voided for Al and AlSi films, while for AlCu films, the percentage void area and the average void size are increased. Volume reduction associated with interfacial TiAl3 formation may be responsible for the increase in void size associated with titanium underlayers.

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